The present invention relates generally to multi-speed transmissions for use with bicycles and, in particular, to an improved variable-ratio transmission of the type wherein the effective diameter of a segmented front drive sprocket can be selectively varied for establishing a series of sequentially ordered speed ratios.
As is common knowledge, the vast majority of commercially-available multi-speed bicycles are equipped with a chain-driven transmission of the type generically referred to as "derailleurs". Traditionally, derailleurs are used in combination with front and rear sprocket sets each having a sequence of progressively sized sprocket rings with each derailleur connected to a shift lever via a suitable cable assembly. When it is desired to change gears, the rider selectively manipulates one or both of the shift levers for actuating the corresponding derailleur which, in turn, guides the chain off of one sprocket ring and onto another of the corresponding sprocket set. As is also known, the size relationship between the diameter of the front and rear sprocket rings establishes the various speed ratios that are available for selection by the bicycle rider to accommodate the varying torque and/or speed requirements that may be encountered.
While multi-speed bicycles equipped with conventional derailleur-type transmissions dominate the commercial market, numerous functional and operational drawbacks exist which severely detract from their popularity. More specifically, derailleurs are made of a large number of delicate parts which require frequent maintenance and adjustment and which can be easily damaged. In addition, derailleur transmission systems are hard to operate since the operator must cognitively decide whether to actuate one or both derailleurs to produce the desired gear change. Also, derailleurs are typically noisy and prone to inefficiency due to chain misalignment and/or the rider's failure to completely derail the chain between sprocket rings. Furthermore, front derailleur systems can not be shifted smoothly while the chain is under load which, in turn, detrimentally impacts the ability of the bicycle rider to smoothly pedal through the terrain or road conditions encountered which necessitated the gear change. This inability to shift with the chain loaded is due largely to the fact that, during the traditional derailing process, the chain must lose contact with the sprocket rings. Finally, it is difficult and cumbersome to remove and subsequently reassembly the rear wheel of a multi-speed bicycle equipped with a rear derailleur-type transmission.
In an effort to overcome the above-noted and other shortcomings associated with derailleur equipped multi-speed bicycles, a plethora of alternative variable-ratio transmissions have been proposed. Most commonly, such alternative transmissions are of a type generically referred to as "variable sprocket" transmissions in which the effective diameter of a segmented front drive sprocket is selectively varied with respect to a constant diameter rear driven sprocket for establishing different speed ratios therebetween. From a pure engineering standpoint, such variable sprocket transmissions are superior to derailleur-type transmissions since they can change speed under power in a very smooth manner and the different speed ratios are evenly sequenced from the highest to the lowest speeds for enhanced ease of operation and rider convenience. Various examples of variable sprocket transmission systems for use in multi-speed bicycles are disclosed in U.S. Pat. No. 4,634,406 to Hufschmid, U.S. Pat. No. 4,696,662 to Gummeringer, U.S. Pat. No. 4,787,879 to Pritchard, and U.S. Pat. No. 4,810,235 to Husted. Unfortunately, each of these variable sprocket transmission systems suffers from one or more inherent disadvantages and/or lacks the degree of engineering refinement and functional simplicity to be commercially successful. Thus, while multi-speed bicycles equipped with derailleurs are difficult to operate, they are still considered better than any variable sprocket transmission currently known.
In view of the above, a need exists to develop a variable sprocket transmission for multi-speed bicycles which overcomes the deficiencies known in the prior art, is relatively simple in structure and yet reliable in operation, and can be economically produced.